A rapid, sensitive, and specific high-pressure liquid chromatographic (HPLC) method for the quantitative analysis of warfarin in plasma is described. The method involves a single solvent extraction of warfarin and the internal standard form acidified plasma, followed by evaporation and HPLC analysis. The method is sufficiently sensitive and accurate for most pharmacokinetic studies: concentrations between 0.1 and 4.0 mug/ml can be measured with a coefficient of variation of 2-5%. Known metabolites of warfarin do not interfere with the analysis.